The general rule seems to be to use a focal length of 90-120. But, you should also know when to break the rules as well. Sure, every portrait should be taken with a 100mm focal length, looking straight at the subject. Try these:

• If your character is a super-hero: move the camera down and point it up at the character. Adjust the focal length to around 60-80. The odd angle and fish-eye-ness will give him a sense of power.
• Alternaticely, move the camera up, adjust the focal length to around 30-50... your character now has a sense of smallness.
• Try lower focal lengths when making images of non-human things. Move the camera right up to the nose of the X-Wing Fighter and adjust the focal length to 25. See what happens.
• Maybe your scene is a close-up of a crazed person. In that case, give him a strange expression and set the focal length to a small value: 10 or 15. Like good photography, think about what your image is saying.
• Is your image a simple "stand and pose" image? In this case, you don't have to worry so much.
• Or is it a "moment in time" image? In this case, you should think about your character's motivation, the background, and the events before the image was "shot". Finally, find some images you really like. Look at how the artist built it: from the composition to the lighting to the colors to the character's expression to the camera angle.